High-molecular polycarbonate foamed plastics are known (see e.g. DE-AS 1,031,507). The foaming agents can be e.g. substances which split off CO.sub.2 or inert gases such as N.sub.2 or CO.sub.2.
Polycarbonates are severely degraded in the presence of amides (see e.g. H. Schnell, Chemistry and Physics of Polycarbonates, Interscience Publishers, New York 1964, page 183).
The use of natural substances for foaming polycarbonates is not taught in the technical literature.